Sharp tunnelling resonance from the vibrations of an electronic Wigner crystal

Photoemission and tunnelling spectroscopies measure the energies at which single electrons can be added to or removed from an electronic system. Features observed in such spectra have revealed electrons coupling to vibrational modes of ions both in solids and in individual molecules3. Here we report the discovery of a sharp resonance in the tunnelling spectrum of a two-dimensional electron system. Its behaviour suggests that it originates from vibrational modes, not involving ionic motion, but instead arising from vibrations of spatial ordering of the electrons themselves. In a two-dimensional electronic system at very low temperatures and high magnetic fields, electrons can either condense into a variety of quantum Hall phases or arrange themselves into a highly ordered ‘Wigner’ crystal lattice. Such spatially ordered phases of electrons are often electrically insulating and delicate, and have proven very challenging to probe with conventional methods. Using a pulsed tunnelling method capable of probing electron tunnelling into insulating phases, we observe a sharp peak with dependencies on energy and other parameters that fit to models for vibrations of a Wigner crystal. The remarkable sharpness of the structure presents strong evidence of the existence of a Wigner crystal with long correlation length.